What it is
E400 is alginic acid, a polysaccharide extracted from brown seaweed (kelp).
Why it matters
Thickener and stabiliser in ice cream, jellies, fruit preserves, salad dressings, and some pharmaceutical preparations. The alginate salts (E401 sodium, E402 potassium, E403 ammonium, E404 calcium) are all vegan too.
What to do
Always vegan.